Globular cluster

“ The distance of these globular clusters from the earth is, however, certainly very great. Attempts to accurately determine their position in space have not been attended with success. As the component stars are at practically the same distance from the eye, we have no comparison stars to measure from, and their exact distance, therefore, remains unknown. We may, however, estimate their probable distance with some show of plausibility. We may assume that the stars of the Hercules cluster would, if concentrated in a point, shine as a star of about the fourth magnitude. ”

“ If gravitation is regnant among the uncounted components of stellar clusters, as doubtless it is, these stars must be in rapid motion, although our photographs of measurements have been made too recently for us to detect even the slightest motion in any of the component stars of a cluster. The only variations are changes of apparent magnitude, of a type first detected in a large number of stars in Omega Centauri, by Bailey of Harvard, who by comparison of photographs of the globular clusters was the first to find variable stars quite numerous in these objects. ”

“ In a recent photograph of this cluster 50,000 stars are shown in an area of the sky which would be entirely covered by the full moon.
Photographs of this swarm of suns fail to do it justice because of its density. In order to give some idea of the wonderful structure of such a cluster a photograph of the globular cluster ω Centauri in the Southern Hemisphere is here shown. It is estimated that this cluster contains in the neighborhood of ten thousand stars, and within its confines Professor Bailey discovered 128 variable stars. ”

“ Groups of Stars.—Great dissimilarity is apparent in the clustering of stars. The heavens furnish us with all gradations—from the loose, open groups like that in Coma Berenices, in the Pleiades, or in Præsepe, to the compact globular clusters, in which some thousands of stars are so densely congregated that considerable optical power is re317quired to disintegrate them. Some, it is true, yield more easily than others. The great cluster (Messier 13) in Hercules readily displays the swarms of stars of which it is composed ”

“ Although these loosely associated star clusters do not show such evidence in favor of family connection as the more closely compacted globular clusters, still we can hardly escape from the conviction that their apparent aggregation is really due to some physical bond of union, and not merely the result of a fortuitous scattering of stars at different distances in the line of sight. ”

“ It seems very probable that if the suns composing the globular clusters appear so near together, it is because, in the first place, they are at immense distances from us, and in the second, because they appear nearly in a line with other suns, which are at a still greater distance from us, and on which they accordingly are nearly projected. ”

“ In open groups, such as the Pleiades, enormous suns are most likely connected with minute self-luminous bodies; but the stars compressed into “globular clusters” appear to be more uniform, and may, perhaps, be intermediate in magnitude. Yet here again, the only thing certain is the prevalence of endless variety. Celestial systems are not turned out by the dozen, like articles from a factory. Each differs from the rest in scale, in structure, in mechanism. Attempts to reduce all to any common standard must then prove futile. ”

“ Perrine of the Lick Observatory finds that (a) “the division of the stars in globular clusters into groups, differing widely in brightness, is characteristic of these objects”; (b) “the globular clusters are devoid of true nebulosity”; and (c) “stars fainter than 15th magnitude predominate in the Milky Way and globular clusters, but elsewhere are relatively scarce.” He found that “exposures of one hour or thereabouts showed as many stars as exposures four to six times as long; the only effect of the longer exposures being in the matter of density.” ”

“ He was thus led to arrange the nebulæ in a presumed order of development. The signs of interior condensation traceable in nearly all, he attributed to the persistent action of central forces. Condensation, then, gave evidence of age. Aggregated stars drew closer and closer together with time. So that scattered or branching formations were to be regarded as at an early stage of systemic existence; globular clusters, as representing universes still in the prime of life ”

“ The result would be that, at some definite distance, and all greater distances, the preponderating light received from such a cluster would be derived from the gaseous envelopes and not from the isolated stellar masses. The spectrum of the cluster would, therefore, become a linear one, like that from the gaseous surroundings of our own sun. ”

“ If, with a telescope of high space-penetrating power, we carefully inspect the firmament, we shall become aware of a belt of clusters—of what we have hitherto called "nebulæ"—a band, of varying breadth, stretching from horizon to horizon, at right angles to the general course of the Milky Way. This band is the ultimate cluster of clusters. This belt is The Universe. Our Galaxy is but one, and perhaps one of the most inconsiderable, of the clusters which go to the constitution of this ultimate, Universal belt or band. ”

“ Star Clusters.—On observing the heavens on a clear, dark night, there can be seen in different parts of the sky closely aggregated groups of stars called clusters. In some instances the component stars are so near together that the naked eye is unable to discern the individual members of the cluster. They then assume an indistinct, hazy, cloudlike appearance. Upwards of 500 clusters are known to astronomers, the majority of which are very remote. ”

“ Each star, if isolated, would be surrounded with its own gaseous envelope. These gaseous envelopes might, in the case of a cluster, form over the whole, or a part, of the cluster a continuous mass of gas. So long as such a cluster was within a certain distance from us, the light from the stellar masses would predominate over that of the gaseous envelope. The spectrum would, therefore, be an ordinary stellar spectrum. Suppose such a cluster to be removed farther and farther from us, the light from each star would be diminished in proportion of the inverse square of the distance ”

“ The "clusters" of which this Universal "cluster of clusters" consists, are merely what we have been in the practice of designating "nebulæ"—and, of these "nebulæ," one is of paramount interest to mankind. I allude to the Galaxy, or Milky Way. This interests us, first and most obviously, on account of its great superiority in apparent size, not only to any one other cluster in the firmament, but to all the other clusters taken together. The largest of these latter occupies a mere point, comparatively, and is distinctly seen only with the aid of a telescope. ”

“ If such be the case, the rule should hold yet more strongly among the stars of a condensed cluster.
Perhaps the most important problem connected with clusters is the mutual gravitation of their component stars. Where thousands of stars are condensed into a space so small, what prevents them from all falling together into one confused mass? Are they really doing so, and will they ultimately form a single body? These are questions which can be satisfactorily answered only by centuries of observation; they must, therefore, be left to the astronomers of the future. ”

“ Star clusters have been arranged into two classes, ‘irregular’ and ‘globular;’ but no sharp line of demarcation exists between them, though each have their distinctive peculiarities. Irregular clusters consist of aggregations of stars brought promiscuously together, and presenting an appearance devoid of any structural arrangement. They are of different shapes and sizes, possess no distinct outline, and are not condensed towards their centre, like those that are globular. ”

“ Lord Rosse describes No. 4511 of Sir John Herschel’s General Catalogue of Nebulæ and Clusters (Phil. Trans., 1864) as “a most gorgeous cluster, stars 12-15 magnitude, full of holes.” [370] His sketch of this cluster shows 3 rings of stars in a line, each ring touching the next on the outside. Sir John Herschel described it as “Cluster; very large; very rich; stars 11-15 magnitude (Harding, 1827) ,” but says nothing about the rings. This cluster lies about 5 degrees south of δ Cygni. ”

“ In the same way certain regions in the sky which are found to be bare of stars would have to be regarded as tunnels through the stellar system. That even one or two such spikes or tunnels should exist would be improbable enough, but as star clusters were known in considerable numbers before Herschel began his work, and were discovered by him in hundreds, it was impossible to explain their existence on this hypothesis, and it became necessary to assume that a star cluster occupied a region of space in which stars were really closer together than elsewhere. ”

“ The distance by which the cluster is separated from the earth renders it impossible for us to see those movements, at all events 55within those narrow limits of time over which our observations have as yet extended; but the laws of mechanics assure us that the mutual attraction of the stars in this cluster must give rise to incessant movements, and that this must be the case notwithstanding the fact that the relative places of the stars in the cluster show no alteration that can be recognised from one year’s end to another. ”

“ One has only to imagine a swarm of such meteors at first hastening busily along their orbit, a great cluster all together, then, by the near neighbourhood of some planet, or by some other disturbing causes, being drawn out, leaving stragglers lagging behind, until at last there might be some all round the path, but only thinly scattered, while the busy, important cluster that formed the nucleus was still much thicker than any other part. ”

“ The solar cluster is shown very dense at the central portion, occupying one-tenth of its diameter, and it is near the outside of this dense centre that our sun is supposed to be situated. Beyond this there seems to be almost a vacuity, beyond which again is the outer portion of the cluster consisting of comparatively thinly scattered stars, thus forming a kind of ring-cluster, resembling in shape the beautiful ring-nebula in Lyra, as has been suggested by several astronomers. ”

“ Cluster work is not altogether unlike "parallax hunting" in its preliminary stage of securing the photographic observations. The object is to obtain an absolutely faithful picture of a star group, just as it exists in the sky. We have every reason to suppose that a very large number of stars condensed into one small spot upon the heavens means something more than chance aggregation. The Pleiades group (page 10) contains thousands of massive stars, doubtless held together by the force of their mutual gravitational attraction. If this be true, there must be complex orbital motion in the cluster ”

“ For each star has to be treated singly and located at its proper distance by elaborate measurements; progress is very laborious. But when we determine the distance of this remote cluster, we secure at one scoop the distances of many thousands of stars. The distance being known, the apparent magnitudes can be turned into true magnitudes, and statistics and correlations of absolute brightness and colour can be ascertained. Even before the distance is discovered we can learn a great deal from the stars in clusters which it is impracticable to find out from less remote stars. ”

“ Suns were flashing by us on either side now, at a rate that was an index to our appalling speed, but still we flashed on with greater and greater speed, racing out between the thronging suns of the galaxy toward its edge, the great ball of suns of the Cancer cluster expanding before us as we raced on in its direction. On—on—until the mighty cluster lay full to our right, until we were flashing past it, the blackness of outer space stretching ahead, and in that far-flung blackness the dim little patch of light that was the Andromeda universe. ”

“ The agreement between the observed and computed log N over a range of more than 8 mag. is consistent with the double assumption of uniform luminosity and uniform distribution or, more generally, indicates that the density function is independent of the distance.
The systematic decrease in the residuals O – C with decreasing luminosity is probably within the observational errors, but it may also be explained as due to a clustering of nebulae in the vicinity of the galactic system. The cluster in Virgo alone accounts for an appreciable part. ”

“ Round Nebulæ and Clusters.—Resolvable nebulæ and333 clusters are frequently circular in outline. The central condensation is an indication of their globular form, though not always so, for many of these objects become suddenly much brighter in the middle, and show an apparently stellar nucleus. The material or stars forming the object cannot therefore be equally distributed. Where it suddenly brightens there is a great condensation, and in some cases several of these are evident in the form of bright rings, intensifying as the nucleus is approached. ”

“ In a future age this may be otherwise. Streams of stars will, perhaps, set outward from these grand assemblages, leaving vacancies behind. Thus, if it be permissible to judge of the relative antiquity of clusters by their advance towards disruption, 47 Toucani and Omega Centauri may be reckoned among the youngest of the globular kind existing in the heavens.
The mechanism of clusters has received little attention from any astronomer beside Herschel. ”